What is Metrology

1.1
The word metrology has a long tradition and is derived from the Greek word for measure.
It is well-known saying that the knowledge about anything is complete only when it can be expressed
in numbers and something is known about it. Thus for every kind of quantity measured, there must
be a unit to measure it and express it in numbers of that unit. Further, in order that this unit is
followed by all and not one who is taking the measurements, there must be a universal standard
and the various units for various parameters of importance must be standardised. Most important
parameter in metrology is the ‘length’ which can be measured in several forms and in several ways.
Measurements play a vital role in every field of investigation and present day scientific and
technological progress has resulted from progress in the field of measurements. In general,
measurements are made to increase our knowledge and understanding of the world with a view to
lead a better life. Measurement science is vital for trade and commerce and is the basis of modern
science and technology.
In metrology, which literally is the science of measurements we have to go one step ahead
and bother ourselves about the correctness of measurement also. We have to see whether the result
is given with the sufficient correctness and accuracy for the particular need or not. Thus we are
primarily concerned with methods of measurement based on agreed units and standards. Metrology
is thus concerned with the establishment, reproduction, conservation and transfer of units of
measurements and their standards. The practice of metrology involves precise measurements
requiring the use of apparatus and equipments (instruments and necessary adjuncts) to permit the
degree of accuracy required to be obtained.
Metrology, the science of measurements, includes all aspects both theoretical and practical
with reference to measurements, whatever their uncertainty, and in whatever fields of science or
technology they occur. Thus metrology is also the science of measurement associated with the
evaluation of its uncertainty. It is important to understand that only to measure is not the specifity
of metrology but the core of metrology lies in the validation of the result, particularly by specifying
its actual limitations. Metrology is not restricted only to standards of length and mass but other
parameters in sectors of social concern, such as health, safety, and environment protection also.
Making mastery of the science of measurement is a prerequisite for progress in science itself.
Industrial manufacturing and many fields of life call for activity at a high scientific and technical
level at which any progress achieved has to be accomplished by progress in metrology. The
increasing automation of manufacture requires the highest level of accuracy. One must remember
the famous saying that man’s knowledge of nature, the universe, and how to adapt nature to his
purpose, advances in step with his ability to measure precisely.
The metrologist has to understand the underlying principles to be able to design and develop
new instruments and also to use the available instruments in the best way. Metrology is therefore
also concerned with the methods, execution and estimation of accuracy of measurements ; the
measuring instruments and the inspectors. Today’s standard of precision and reliability are so high
that man’s basic instincts and senses are inadequate to cope with them’. To this end, use has to be
made of precision measuring instruments and various types of conventional and sophisticated
gauges and comparators.
Thus, it can be said that metrology is mainly concerned with (i) establishing the units of
measurements, reproducing these units in the form of standards, and ensuring the uniformity of
measurements, (ii) developing methods of measurement, {Hi) analysing the accuracy of methods of
measurement, establishing uncertainty of measurement, researching into the causes of measuring
errors, and eliminating these.
In the broader sense, metrology is not limited to length measurement but is also concerned
with the industrial inspection and its various techniques. Due to big industrial revolution and great
advancement, industrial inspection does not simply mean the fulfilling of the specifications laid
down by the manufacturers. Rather inspection in real sense is concerned with the checking of a
product at various stages in its manufacture right from the raw material form to the finished
products and even assembled parts in the form of machine also. Inspection is carried out with gauges
and the metrologist is intimately concerned with the design, manufacturing and testing of gauges
of all kinds. Dynamic metrology is concerned with measuring small variations of continuous nature.
The measurement science today has developed to electronically operated and controlled equipments,
computer-aided systems for on-line monitoring, opto-mechanical, laser and fibre optics based
instruments, etc.
As regards length measurement (dimensional inspection), we will be dealing with non-
precision and precision linear measurements and study the various instruments used for this
purpose. The standardisation of various units is also important and we will study what are the
various standards for linear measurements and how attempts are made to preserve and maintain
these standards. We will also see how the light wave standard helps us in doing away with material
standards. For very precise measurements, methods based on light wave interference phenomena
will also form a separate chapter.
Metrological activities start from establishment of measurement standards, appraisal of
various physical parameters including dimensions, development of measuring instruments and
techniques, and calibration of test and measurement equipments. All this is essential for correct
operative measurement for quality and products and services delivered by the industry. Present
day industry demands not only one time achievability, but aims for conformity involving such
aspects as repeatability, reproducibility, interchangeability, of very many dimensions and charac-
teristics and evidence thereof, for confidence of both producers and customers. This is possible by
creation of standards and measurement techniques.
Due to mass production, it can be very easily realised that it is not possible to measure the
various elements of a component by conventional methods. Thus other devices, i.e. gauges and
comparators will be studied in detail. Further it is also not advisable to measure all the components
in mass production if they are coming out of automatic machines. It will be seen that inspection of
a few components out of a big lot is sufficient under the study of statistical quality control, the
knowledge of which is very essential now-a-days.
As regards assembly and fitting of various components, some system of limits and fits has
to be followed throughout and we will study the Indian Standard for ‘Limits and Fits’. For assembled
products in form of machine, it is essential that the relative movements of various parts of machine
take place in a desired way. For this purpose study of machine tool alignment tests is very essential.
Many times in actual production, angle measurement presents a big problem and its
thorough understanding and various techniques involved in it and circular division form an
important part of metrology. Sometimes we come across various measurements which are really
quite typical and problems of this type can be easily solved with the help of some trigonometrical
relations. Such measurements will be studied in the chapter of Miscellaneous Measurements.
It may be emphasised here that man has to handle various instruments and sense of feel’
plays very important role. In order that all people get same readings for a component by the same
instrument, the instrument should be designed in such a way that always constant pressure is
applied between the component and the instrument. Also instrument must be held such that the
sense of ‘feel’ present in hand is free to give correct decision. However, in Universal Machines, an
attempt is made to eliminate human errors due to different senses of touch and feel.
Good machines and their proper functioning calls for very good finished surfaces and thus
study of the surface finish and various methods to estimate it quantitatively is very essential. A
most commonly used tool in the hand of metrologist is ‘dial-indicator’ which will be studied in detail
separately. Testing of gauges and dynamic measurements also deserve full attention.
We will also be dealing with the screw thread measurement and gear measurement and
gauges for screw threads, as these are most popular parts one comes across in a workshop and
machines.
A chapter is also devoted to non-destructive testing of metals and alloys to make one familiar
with those methods which do not destroy the material and at the same time check all the desired
properties and examine the internal structure of materials for homogeneity.
There is a great awareness about quality and industries are adopting the approach of Total
Quality Management. Chapters have thus been devoted to Quality Assurance Programmes and
Total Quality Management with reference to ISO 9000. A chapter is also devoted to Machine Vision
Systems.
In broader sense, metrology (the science and art of precision measurement, testing and
evaluation) is the mother science for technological development. The advancements in industry
depend, to a great extent, on the quality and reliability of dimensional accuracy and precision
measurement of other physical characteristics.1.1.1.

Legal Metrology.

Legal Metrology is that part of metrology which treats units of
measurement, methods of measurement and the measuring instruments, in relation to the
statutory, technical and legal requirements. It assures security and appropriate accuracy of
measurements. Lack of legislation regarding various measures will lead to great uncertainly.
Legal metrology is directed by a national organisation, viz. National Service of Legal
Metrology whose object is to resolve problems of legal metrology in a particular country. Its functions
are to ensure the conservation of national standards and to guarantee their accuracy by comparison
with international standards ; and also to impart proper accuracy to the secondary standards of
the country by comparison with international standards.
The contemporary organisation of metrology includes a number of international organisa-
tions viz. (a) The International Organisation of Weights and Measures : and (6) National Service
of Legal Metrology whose ultimate object is to maintain uniformity of measurements throughout
the world.
The activities of the service of Legal Metrology are : control (testing, verification, stand-
ardisation) of measuring instruments ; testing of prototypes/models of measuring instruments ;
examination of a measuring instrument to verify its conformity to the statutory requirements, etc.
Legal metrology has application in :
(i) Commercial transactions (net quantity)
(ii) Industrial measurements (proper control on accuracy of measurement, so as to ensure
interchangeability with a view to promoting mass production.
(iii) Measurements needed for ensuring public health and human safety.
A national law relating to legal metrology covers the following points :
(i) Legal units of measurements. In 1976, Parliament enacted comprehensive law, the
Standards of Weights and Measures Act 1976, to establish the International System of Units (SI),
to regulate inter-State trade or commerce in weights and measures, and to provide for other matters
important from the view-point of consumer protection.
(ii) physical presentation of legal units ;
(Hi) hierarchy of standards—their maintenance and custody;
National Standards (Echelon-I)
Reference Standards (Echelon-II)
Secondary Standards (Echelon-Ill A)
Working Standards (Echelon-Ill B)
(iv) specifications or technical regulations of measuring instruments as regards their
metrological and technical requirements ;
(v) metrological control on measuring instruments ; (approval of model, initial verification,
periodical verification ; verification after repairs, inspection of the use of measuring instruments)
(vi) metrological control on pre-packed goods ;
(vii) control on manufacture, repair and sale of measuring instruments ;
(viii) organisation/service concerned with legal metrology ;
(ix) levy and collection of fees ;
(x) penalties for contraventions ;
(xi) training of personnel.1.1.2.

Deterministic Metrology.

This is a new philosophy in which part measurement is
replaced by process measurement. In the deterministic metrology, full advantage is taken of the
deterministic nature of production machines (machines under automatic control are totally deter-
ministic in performance) and all of the manufacturing sub-systems are optimised to maintain
deterministic performance within acceptable quality levels. In this science, the system processes
are monitored by temperature, pressure, flow, force, vibration, accoustic “finger printing” sensors,
these sensors being fast and non-intrusive. The new technique such as 3D error compensation by
CNC (Computer Numerical Control) systems and expert systems are applied, leading to fully
adaptive control. This technology is used for very high precision manufacturing machinery and
control systems to achieve microtechnology and nanotechnology accuracies.